Cu3Sn joint based on transient liquid phase bonding of Cu@Cu6Sn5 core–shell particles

With the development of high-integration and high-power electronics, the lack of matching chip connecting materials that can withstand high temperatures has been a challenge. In this manuscript, a Cu@Cu6Sn5 core–shell bimetallic particles (approx. 1 μm in diameter) are successfully prepared and introduced as a new solder material for the packaging of power devices to obtain a Cu3Sn all-IMC solder joint. The joint consisted mainly of equiaxed Cu3Sn grains, and a small portion of columnar Cu3Sn grains. In columnar-type growth, Sn is the dominant diffusing species, which comes from the depletion of Sn in Cu6Sn5. The depleted Cu6Sn5 is transformed into columnar Cu3Sn. In equiaxed-type growth, Cu is the dominant diffusing species. Cu reacts with Cu6Sn5 to grow a Cu3Sn layer. This conclusion was confirmed by the orientation relationship. The equiaxed Cu3Sn grain nucleates at the Cu/Cu3Sn interface have an orientation relationship with the Cu substrate. Columnar Cu3Sn grains at the Cu6Sn5/Cu3Sn interface have an orientation relationship with Cu6Sn5.


Materials and methods
Cu@Cu 6 Sn 5 particles. To prepare the Cu@Cu 6 Sn 5 core-shell particles, Cu particles (approx. 1 μm in diameter) with a particle size of 0.5-1.0 μm were used. A specific amount of cleaned Cu Particles and polyethylene glycol were dispersed completely in deionized water. Then, a reducing agent constituting sodium citrate, sodium hypophosphite, hydroquinone, and disodium EDTA in a mass ratio of 10:30:1:1 was added to the solution. Subsequently, a ligand CH 4 N 2 S was added to the solution. The amount of CH 4 N 2 S was adjusted such that the mass ratio of CH 4 N 2 S to Cu remained between 3:1 and 2:1. In another container, stannous chloride dihydrate was added to hydrochloric acid, followed by ultrasonication until the solution was clarified and transparent. The amount of stannous chloride was adopted such that the mass ratio of stannous chloride to Cu remained between www.nature.com/scientificreports/ 1:2 and 1:3. The stannous chloride solution was then added to the solution containing Cu Particles and stirred continuously for 50-90 min at room temperature to ensure a complete reaction. The reaction product was separated from the solution, repeatedly cleaned, and dried. The chemical reaction is as below: The heat given off by the reduction reaction accelerates this reaction (Fig. 2). The particles were characterized by XPS (Thermo, Scientific K-Alpha), SEM (FEI, FIB/SEM; HELIOS 600i), EDS (EDAX, XM4) and XRD (Rigaku, D/max 2800).
Soldering. Ethyl cellulose and dibutyl phthalate were added to the pine oil alcohol solution and mixed under assisted sonication for 1 min. Then, a mixture of Span-85 and sulfosalicylic acid was added dropwise to the solution. The pine oil alcohol solution was mixed with Cu@Cu 6 Sn 5 particle and SAC305 particles at a mass ratio of 2.8:1 to obtain a paste, which at this ratio, the atomic ratio of Cu to Sn in the paste is 3.2:1. The paste was screenprinted on the surface of a Cu substrate and reflowed at 280 °C under a pressure of 10 MPa for 60 min (Fig. 3a).
It is worth noting that auxiliary pressure is necessary in the welding process because the process of Cu reacting with Cu 6 Sn 5 to generate Cu 3 Sn is accompanied by volume shrinkage, which results in voids. Additional pressure is required to reduce the number of voids. Based on the TLP bonding, the SAC305 melt filling reacts with the Cu-nuclei to generate Cu-Sn intermetallic compounds (IMCs) by heating and pressurizing. This reaction consumes the low-melting-point Sn phase and produces high-temperature solder joints. The bend surface of molten Sn is subjected to a certain additional pressure on the surface under the effect of surface tension.
The Cu-Sn interfacial chemical reaction is expressed as The rate of change in Gibbs free energy is the highest when the products adopt a scallop shape, which is favorable for the reaction. Therefore, Cu 6 Sn 5 shows a scallop-type morphology. Considering the liquid solder during the soldering reaction as a binary solution system, where Cu is the solute and Sn is the solvent, the distribution of Cu in the liquid solder satisfies the Gibbs-Thomson effect. The difference in Cu concentration serves as the driving force for the diffusion of Cu in the soldering reaction, and the diffusion of Cu between adjacent IMC grains with different radii of curvature also leads to the incorporation of adjacent IMC grains. The microstructure undergoes a phase transformation in the order of Cu@η-Cu 6 Sn 5 → ε-Cu 3 Sn. Eventually, the joint loses the typical characteristics of a core-shell structure and instead forms a uniform microstructure, as shown in Fig. 3. The microstructure of solder joints and shear fracture surfaces were characterized using a focused ion beam/ scanning electron microscope (FIB/SEM; HELIOS 600i; FEI) equipped with an electron dispersive X-ray detector (EDX; XM4; EDAX). The composition of shear fracture surfaces was characterized by X-ray diffractometry (XRD; D/max 2800; Rigaku). The melting points of the different phases in the solder joints were measured with Sn + + Cu + 2(CH 4 N 2 S) = Sn + [Cu(CH 4 N 2 S) 2 ] 2+ 6Cu + 55n = Cu 6 Sn 5 6Cu + 5Sn → Cu 6 Sn 5 Cu 6 Sn 5 + 9Cu → 5Cu 3 Sn www.nature.com/scientificreports/ a differential scanning calorimeter (DSC; STA 449F5; NETZSCH) at a heating rate of 10 °C s −1 . The morphology of joint/Cu interface was observed by transmission electron microscopy (TEM, TecnaiG2F30, FEI). And the grain orientation and grain sizes distribution of Cu 3 Sn was analyzed by Electron Backscattered Diffraction (EBSD, Nordly max3, Oxford).
To verify the long-term service reliability of the solder joints at high temperatures, the samples were subjected to aging tests at 300 °C using a muffle furnace, and the joint and mechanical properties of the samples were examined at 300, 600, 900, and 1200 h, respectively. A creep tester (SANS, GWTA-105, 100 kg) was used to measure the shear strength of the welded joints at room temperature at a shear rate of 0.25 mm s −1 . The sheared sample is a 5 × 5 × 2 (mm) copper substrate soldered to a 10 × 10 × 2 (mm) copper substrate (Fig. 3).

Results and analysis
Cu@Cu 6 Sn 5 particles. Statistically, the diameter length of the particles is mainly distributed between 0.5 and 1.3 μm (Fig. 4a,b). The XRD pattern results of the particles showed that the surface of the particles is η-Cu 6 Sn 5 , and the EDX results also support this conclusion (Fig. 4c). SEM images show that Cu 6 Sn 5 on the surface exhibits a scallop-like character (Fig. 4e). After chemical plating, the scallop-shaped shell covers the surface of the smooth Cu core. As shown in Fig. 4f, an EDX scan analysis is performed at the Cu-Cu 6 Sn 5 interface, the average diameter of the Cu particles was 600 nm, and the thickness of the shell is about 200 nm (radial difference), Cu atoms diffuse throughout the shell (Fig. 4d). Because smaller copper particles have higher surface activity energy, the chemical reaction between the Cu core and the Sn layer is accompanied in the process of electroless Sn plating to generate Cu 6 Sn 5 .
Evolution of the joint in soldering. The microstructure undergoes a phase transformation in the order of Cu@Cu 6 Sn 5 + SAC305 → Cu@Cu 6 Sn 5 + Cu 3 Sn → Cu 3 Sn. Eventually, the joint loses the typical characteristics of a core-shell structure and instead forms a uniform microstructure. The joint reflowing 30 min and 60 min are analyzed using Scanning SEM coupled with energy dispersive X-ray spectroscopy (EDX) to confirm the transformation of the binary system (Fig. 5). The EDX results demonstrate the process of second stage diffusion.
During the reflow of the Cu@Cu 6 Sn 5 core-shell particles with SAC305, the reaction takes place in two stages. First stage, the SAC305 reacts with Cu to form Cu 6 Sn 5 IMCs. In this stage, Cn atoms diffuse from the core-shell particles through the Sn melt throughout the joint and react with the Sn melt to form Cu 6 Sn 5 . The Cu 6 Sn 5 nucleation event occurs at the solid-liquid phase interface, i.e., the Cu 6 Sn 5 /Sn interface. The growth of Cu 6 Sn 5 at this stage is dominated by grain boundary diffusion. The grain boundary diffusion is very fast, so the Sn melt disappears very quickly. In experiments, that auxiliary pressure and high temperatures accelerate the process (280 ℃, 10 Mpa), it only takes about 5 min (Fig. 6a) and there is almost no residual Sn melt in the joint.
In the second stage, The Sn atoms also diffuse, and the Cu atoms continue to diffuse. The remaining Cu atoms diffuse and react with Cu 6 Sn 5 to form Cu 3 Sn. In this joint, the Cu 3 Sn nucleation events are more complex compared to the conventional sandwich TLP method (Figs. 1, 8a,b). Nucleation of Cu 3 Sn occur over two interfaces, the Cu/Cu 6 Sn 5 interface, and the Cu 6 Sn 5 /Cu 3 Sn interface, respectively. The nucleation at different interfaces www.nature.com/scientificreports/ results in different Cu 3 Sn grain morphologies. The number of equiaxed grains is much higher than that of columnar grains in this joint obtained by having Cu@Cu 6 Sn 5 reflow. Thus, the number of columnar crystals is positively correlated with the percentage of SAC305 in the solder paste. Two different morphologies, one with equiaxed grains (Figs. 6, 7) and one with columnar grains (Fig. 6), are obtained by observing electron backscattering diffraction (EBSD) mapping with different nucleation interfaces. In Fig. 7b, the Cu 3 Sn grains on the top side near the Cu/Cu 3 Sn interface are equiaxed grains, while those on the bottom side near the Cu 3 Sn/Cu 6 Sn 5 interface are columnar grains. The TEM mapping and electron diffraction pattern results (Figs. 8,9) show that the two morphologies of Cu 3 Sn have the same crystal structure. The grains of both morphologies have the same crystal structure-the space group of the crystal is cmcm(63).
Two different morphologies of Cu 3 Sn. Two different morphologies of Cu 3 Sn are observed in the experiment, equiaxed and columnar (Fig. 6). The Cu 3 Sn phase arises from a solid-state reaction between Cu and Cu 6 Sn 5 , which is diffusion controlled. The reaction of the Cu-Sn binary system is controlled by the Gibbs free energy change rate, and the reaction path of the system tends to have the largest Gibbs free energy change rate (�G) 20,21 .
A further study by Paul 22 updated the ratio of Cu@Cu 6 Sn 5 interdiffusion coefficients and found that in Cu 3 Sn, Cu is the dominant diffusing particle, while in Cu 6 Sn 5 , the diffusion of Sn is slightly faster than Cu in Cu 6 Sn 5.
The growth of equiaxed Cu 3 Sn grains is a ripening process, which is dominated by the diffusion of copper atoms from the copper substrate to the Cu 6 Sn5/Cu 3 Sn interface to form Cu 3 Sn 7,8,12,14 . Refs. 23,24 after analyzing their systematic experimental data on retarded Cu 3 Sn formation, concluded that "nucleation rather than growth is the cause of Cu 3 Sn deficiency." This new insight distinguishes from all previous studies on Cu 3 Sn fabrication,  www.nature.com/scientificreports/ which focused on stimulating Cu 3 Sn growth rather than nucleation. Thus, it provides a new fundamental clue to the fabrication of Cu 3 Sn. Simulation of several Cu3Sn superstructures reveals that the presence of anti-phase boundaries can change the transport anisotropy by ~ 10%. The DFT thermodynamic stability analysis suggests that the previously observed D019 structure featuring the maximum number of anti-phase boundaries is the Cu 3 Sn ground state in the relevant temperature range, which points to the importance of kinetic factors in the formation of the known long-period superstructures 7,22,24 . The Cu 3 Sn grains appearing along the Cu/Cu 6 Sn 5 interface were found to have different grain orientations by electron backscatter diffraction techniques (Fig. 7). On this basis, these equiaxed Cu 3 Sn grains formed after nucleation also have different grain orientations. The orientation of a grain depends on the arrangement of the atoms within that grain. This means that the atomic arrangements between equiaxed Cu 3 Sn grains are different. Due to the difference in atomic arrangement, equiaxial Cu 3 Sn grains need to grow in different directions to obtain the lowest energy. However, for each equiaxed Cu 3 Sn grain, its growth along the preferred growth direction is hindered by its neighboring Cu 3 Sn grains. Although the growth of Cu 3 Sn grains along their preferred growth direction is prevented, the growth of Cu 3 Sn grains does not stop. This means that the Cu 3 Sn grains must grow in other ways. Initially, the Cu 3 Sn grains seek to grow in other directions. Of course, more energy is required to grow along these non-preferred directions. However, the possibility exists that the energy required to nucleate a new Cu 3 Sn grain shape may be lower compared to the energy required to grow along these non-preferred directions as well as to grow in other ways 14,25 .
The diffusion rate of Cu atoms in Cu 6 Sn 5 is much smaller than the diffusion rate of Cu atoms in Sn. Therefore, when Cu 3 Sn is nucleated at the Sn/Cu 6 Sn 5 interface, the growth of Cu 3 Sn is dominated by the diffusion of Sn atoms, and Cu 3 Sn grows in the direction of the lowest energy. In contrast, when Cu 3 Sn is nucleated at the Cu/ Cu 6 Sn 5 interface, the Sn atom diffusion dominates the growth of Cu 3 Sn, which is more likely to grow along all isometric directions and is easily nucleated. The SAC305 melt provided fast diffusion channels for Sn atoms as well as Cu atoms during the pre-reaction stage. The Cu@Cu 6 Sn 5 particles provided many Cu/Cu 6 Sn 5 interfaces, allowing equiaxed grains to form rapidly and making it difficult to grow. In a small number of regions in the www.nature.com/scientificreports/ joint, the enrichment Cu 6 Sn 5 results in the formation of columnar grains at the Cu 6 Sn 5 /Cu 3 Sn interface. That is, Cu 3 Sn tends to nucleate more at the Cu/Cu 6 Sn 5 interface, while Cu 3 Sn nucleated at the Cu 6 Sn 5 /Cu 3 Sn interface tends to grow into columnar grains.
In columnar-type growth, Sn is the dominant diffusing species, which comes from the depletion of Sn in Cu 6 Sn 5 . The depleted Cu 6 Sn 5 is transformed into columnar Cu 3 Sn. In equiaxed-type growth, Cu is the dominant diffusing species. Cu reacts with Cu 6 Sn 5 to grow a Cu 3 Sn layer. In this process, the equiaxed grains grow in preference to the columnar grains. Different diffusion modes of different atoms affect the lattice type and change the morphology of Cu 3 Sn. We confirmed this conclusion by observing the orientation relationship.
Orientation relationship. TEM mapping is performed on the joints obtained after reflowing of two solder paste, respectively. One is solder joint after reflowing at 280 °C for 60 min (Cu/Cu 3 Sn interface Fig. 8), and the other is solder joint after reflowing at 280 °C for 30 min (Cu/Cu 6 Sn 5 /Cu 3 Sn interface Fig. 9).
Cu 3 Sn was reported as an ɛ-phase with a Cu 3 Ti-type 26 . The electron diffraction pattern is taken from the Cu 3 Sn equiaxed grains phase in different directions (Fig. 8). In the diffraction pattern, the stronger spots correspond to the main reflections of the basic hexagonal lattice, while the weaker additional spots, appearing at half the distance between the main reflections, correspond to the superlattice reflections of the superstructure of the basic hexagonal lattice. The equiaxed Cu 3 Sn grains have an orientation relationship with the Cu substrate, and the orientation of the Cu substrate affects the Cu 3 Sn grains nucleated at the Cu/Cu 6 Sn 5 interface. The electron diffraction pattern shows site-orientation relationships: Cu [− 1 1 0]//Cu 3 Sn [− 1 1 − 3] (Fig. 8g), Cu [− 1 2 1]// Cu 3 Sn [− 1 2 0] (Fig. 8h,i), Cu [1 1 1]//Cu 3 Sn [1 1 2] (Fig. 8k). Caused by the poor interdiffusion coefficients of Cu atoms in Cu 3 Sn, a large number of Cu atoms gather and accumulate at the Cu 3 Sn/Cu interface (Fig. 11c,f), and the lattice structure on the copper side has also been damaged (Fig. 8m). This confirms the nucleation of equiaxed Cu 3 Sn grains at the Cu/Cu 6 Sn 5 interface, dominated by the diffusion of Cu atoms.
There is no orientation relationship observed either between equiaxed Cu 3 Sn grains and columnar Cu 3 Sn grains (Fig. 9i,j). This suggests that the columnar Cu 3 Sn grains is developed thermally during the solidification of the Cu-Sn alloy. However, the columnar Cu 3 Sn grains have a orientation relationship with the Cu 6 Sn 5 grains: Cu 6 Sn 5 [1 0 2]//Cu 3 Sn [0 0 2],:Cu 6 Sn 5 [1 5 2]//Cu 3 Sn [1 4 2]. The above orientation relationship once again confirms our proposed hypothesis that in columnar-type growth, Sn is the dominant diffusing species, which comes from the depletion of Sn in Cu 6 Sn 5 . The depleted Cu 6 Sn 5 is transformed into columnar Cu 3 Sn. The antiphase boundaries (APB) structure was observed in the columnar Cu 3 Sn region (Fig. 9i,k). The antiphase boundaries can be described as larger orthorhombic unit cells with extended dimensions in the b-axis. APBs in columnar Cu 3 Sn crystals are observed. This is because the APB superstructure is based on the Cu 3 Ti-type lattice, which is orthorhombic.
Shear strength and fracture. Shear experiments (Fig. 10) revealed that the shear strength of the joint is approximately 63.2 MPa and 65.2 MPa at room temperature and 300 °C, respectively. The strength of this joint is stronger than those made with most of the current soldering joint materials (SAC-305, Sn-Bi, etc.) and much higher than their service temperatures. www.nature.com/scientificreports/ Notably, the formation of Cu 3 Sn is often accompanied by volume shrinkage, and hence, the Cu 3 Sn phase often contains numerous cavities. An auxiliary pressure of 10 MPa is applied to the joint during the soldering process, which significantly reduced the number of voids in the joint. In the aging experiments at 300 °C, the organization and properties of the joints remained unchanged even after 600 h. The shear fracture of the unaged sample is analyzed. The fracture cross section is mainly composed of equiaxed Cu 3 Sn grains (Fig. 11a,c,e), and the fracture mode is plastic intergranular fracture, and shear band tape exists on the fracture surface (Fig. 11b,f). Columnar Cu 3 Sn grains are also found in the fracture, distributed only in a very small area. Under shear stress, the plastic deformation of Cu 3 Sn grains is highly localized, forming micron-scale shear bands; the formation and rapid expansion of shear bands induce macroscopic brittle fracture of the joint (Fig. 11d).

Conclusions
Cu@Cu 6 Sn 5 core-shell particles (1 μm) are prepared by the chemical reduction method.
A solder paste is obtained by mixing Cu@Cu 6 Sn 5 particles with SAC305 in a 2.8:1 mass ratio and adding pine oil alcohol. This solder paste is reflowed at 280 °C and 10 MPa auxiliary pressure for 60 min to obtain a joint composed entirely of Cu 3 Sn. The joint consisted mainly of equiaxed Cu 3 Sn grains, and a small portion of columnar Cu 3 Sn grains. The reason why the joints are mainly composed of equiaxed Cu 3 Sn grains is because the Cu@Cu 6 Sn 5 particles provide enough Cu/Cu 6 Sn 5 interface.
In columnar-type growth, Sn is the dominant diffusing species, which comes from the depletion of Sn in Cu 6 Sn 5 . The depleted Cu 6 Sn 5 is transformed into columnar Cu 3 Sn. In equiaxed -type growth, Cu is the dominant diffusing species. Cu reacts with Cu 6 Sn 5 to grow a Cu 3 Sn layer.  www.nature.com/scientificreports/

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.  www.nature.com/scientificreports/